Communication devices such as terminals or wireless devices are also known as e.g. User Equipments (UEs), mobile terminals, wireless terminals and/or mobile stations. Such terminals are enabled to communicate wirelessly in a wireless communication system or a cellular communications network, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two wireless devices, between a wireless device and a regular telephone and/or between a wireless device and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the wireless communications network.
The above terminals or wireless devices may further be referred to as mobile telephones, cellular telephones, laptops, or tablets with wireless capability, just to mention some further examples. The terminals or wireless devices in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by an access node such as a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. an “eNB”, an “eNodeB”, a “NodeB”, a “B node”, or a Base Transceiver Station (BTS), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated at the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals or wireless devices within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
A Universal Mobile Telecommunications System (UMTS) is a third generation (3G) telecommunication network, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code-division multiple access (WCDMA) and/or High-Speed Packet Access (HSPA) for user equipment. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third and higher generation networks, and investigate enhanced data rate and radio capacity. In some RANs, e.g. as in UMTS, several radio network nodes may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural radio network nodes connected thereto. This type of connection is sometimes referred to as a backhaul connection. The RNCs and BSCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System (EPS), also called a Fourth Generation (4G) network, have been completed within the 3GPP and this work continues in the coming 3GPP releases, for example to specify a Fifth Generation (5G) network. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long-Term Evolution (LTE) radio access network, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access network wherein the radio network nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of an RNC are distributed between the radio network nodes, e.g. eNodeBs in LTE, and the core network. As such, the RAN of an EPS has an essentially “flat” architecture comprising radio network nodes connected directly to one or more core networks, i.e. they are not connected to RNCs. To compensate for that, the E-UTRAN specification defines a direct interface between the radio network nodes, this interface being denoted the X2 interface.
In the 3GPP LTE, base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
The 3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
Multi-antenna techniques may significantly increase the data rates and reliability of a wireless communication system. The performance is in particular improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a Multiple-Input Multiple-Output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO systems.
Beamforming will be an important technology in future radio communication systems since it may improve performance. The performance may be improved both by increasing the received signal strength, thereby improving the coverage, and by reducing unwanted interference, thereby improving the capacity. Beamforming may be applied both in a transmitter and a receiver. In a transmitter, beamforming amounts to configuring the transmitter to transmit the signal in a specific direction or in a few directions, and not in other directions. In a receiver, beamforming amounts to configuring the receiver to only receive signals from a certain direction or from a few directions, and not from other directions. When beamforming is applied in both the transmitter and the receiver for a given communication link, we refer to the link as a Beam Pair Link (BPL) comprising a combination of beams selected in the both ends. A BPL may also be referred to as a Beam-Tracking Process (BTP) since it may be subject to different procedures for its maintenance. Generally, the beamforming gains are related to the widths of the used beams. For example, a relatively narrow beam provides more gain than a wider beam.
FIG. 1 is a combined flowchart and signalling scheme schematically illustrating a normal BPL setup according to prior art. As shown in FIG. 1, a UE transmits a detected Beam Reference Signal (BRS) report to the network (NW), e.g. to a network node. The report comprises information relating to newly detected network beams based on received reference signals from the NW. The NW instructs the UE to start searching for suitable UE beams, to set up a BPL (referred to as tracking process set up in FIG. 1), and to report a tracking set up completion back to the NW. The NW starts sending dedicated BRS or continues to send BRS.
For a more general description of beamforming, one typically talks about “beamforming weights” rather than “beams”. On the transmission side, the beamforming weights are the complex constants that the signal to be transmitted are multiplied with before being distributed to the individual antenna elements. There is a separate beamforming weight for each antenna element, which allows maximum freedom in shaping the transmission beam given the fixed antenna array. Correspondingly, on the receiving side, the received signal from each antenna element is multiplied separately with the beamforming weights before the signals are combined. However, in the context of the present text, the description is easier to follow if the somewhat simplified notion of beams, pointing in certain physical directions, is adopted.
Beamforming requires some form of beam management, such as beam search, beam refinement, and/or beam tracking, to determine what transmit and receive beams, e.g. directions, to use for communication between a transmitter and a receiver. Beam search may involve the transmitter sweeping a signal across several beams, to allow a receiver in an unknown direction to receive the signal. Beam search may also involve the receiver scanning across several receive beams, thereby being able to receive a signal from an initially unknown direction. Beam search typically also involves the receiver sending a message to a transmitter to indicate which transmit beam or beams are best suited for transmission to that receiver.
Beam refinement and/or tracking is applied when a working beam or a beam pair is already selected. Beam refinement is to improve an already selected beam, for instance changing its beamforming weights such that a narrower beam that provides a better gain is obtained. Beam tracking is to update the selected beams, i.e., to replace the Tx- or Rx-beam in an existing BPL when the conditions change, e.g., due to mobility. Beam refinement and tracking are typically performed by temporarily evaluating a different beam than the one that is currently used for communication, and switching to that beam if it is deemed better than the current.
Beam search may take considerable time, if there are many beams to search for on both the transmitter and receiver side, and during this time communication is typically not possible. Beam refinement and tracking, on the other hand, are usually ongoing activities that cause little or no disturbance to ongoing communication.
Communications networks, e.g. by means of a network node, may transmit periodic or continuous reference signals that are semi-statically configured to support mobility and beam management, e.g. by sweeping across several transmit beams as described above. Such transmissions are here referred to as Beam Reference Signals (BRS) or Mobility Reference Signal (MRS). It is here envisioned that some aspects of beam management may then be performed by a terminal with little or no explicit involvement from the network, if the terminal may assume that the network is transmitting the BRS periodically or continuously. For instance, in some candidate realizations of 5G, terminals perform beam search as part of the system-acquisition procedure, resulting in the selection of a terminal beam such that by using this beam the terminal is able to sufficiently well receive BRS transmitted on a certain network beam. Then the terminal performs a random-access transmission using its selected terminal beam using a transmission resource, e.g. a time and/or a frequency resource, where it expects the network to be able to receive random-access transmissions using that certain network beam. Terminals may continue to receive BRS even when communication is ongoing, to search for new communication paths and to perform refinement and tracking of currently used beams.
Many radio communication systems include some kind of radio-link supervision, whereby the quality of the communication is regularly evaluated, and some action is taken in case the quality is unacceptable or the communication is lost. Radio-link supervision often involves a receiver checking the presence and/or evaluating the quality of a sync signal or a reference signal. It can also involve monitoring the number of retransmissions in a retransmission protocol, and monitoring the time it takes to receive a response to an earlier transmitted request message. In case any of these evaluations indicate a severe problem, the terminal often declares a radio-link failure and initiates some action. In case of a network node having lost communication with a terminal, the action can involve releasing some or all network resources related to that terminal. In case of a terminal having lost communication with a network, the action may involve searching for sync and reference signals from the network and, in case such signals are found, attempting to access the network again. In a beamforming system, this typically involves beam search.
In addition, communications networks schedule and transmit terminal-specific reference signals that, among other things, can be used for beam searching, beam tracking, and beam refinement. Such signals are here referred to as dedicated Beam Reference Signals (BRS) or Beam-Refinement Reference Signals (BRRS). Another example of a terminal-specific reference signal is the Channel-State Information Reference Signal (CSI-RS). This is a reference signal scheduled by the network for one (or possibly, several) specific terminal (or terminals) with the intention of providing measurement opportunities in the terminal such that more detailed channel knowledge may be obtained and reported back to the network.
Finally, communications networks schedule reference signals transmitted in the UL that, among other things, also may be used for beam searching, beam tracking, and beam refinement. Such signals are here referred to as Sounding Reference Signals (SRS).
To sustain a transmission link between the communications network, e.g. the network node, and the terminal over time-varying conditions (e.g. due to mobility) terminals typically consider several possible BPLs for which the beams are tracked and refined. Such BPLs that are identified jointly by the network and the terminal are here referred to as monitored BPLs.
Out of the monitored BPLs, the communications network and terminal agree to use at least one BPL for data and control channel reception and transmission, herein referred to as an active BPL. Depending on its capabilities, a terminal may support one or more active BPL. Whether two BPLs may be simultaneously active or not depends on the terminal implementation. If the terminal-side (UE) beams associated with two BPL are realized using the same processing components such as antenna panels, analog and/or digital circuitry, software units, etc., the UE may not be able to transmit and receive using those UE beams simultaneously. If that is the case, the BPLs are regarded as incompatible. Whether BPLs are compatible or not has to be known by the communications network, since it typically selects which BPLs to be active or monitored. This is solved during the initialization of the BPL based on a compatibility indication from the UE to the network.
Tracking a BPL implies beam tracking and/or beam refinement at the communications network as well as the terminal. To track a BPL (active or monitored) there must be some transmissions on which to measure and evaluate the link quality. In DL, the more persistent BRS may enable tracking of the DL Tx-beam and, more slowly, of the DL Rx-beam. For faster DL Rx-beam tracking scheduled BRRS may be used. In the event of DL/UL reciprocity, the BRS may be sufficient to track a BPL and no UL transmissions are thus needed.
As described above, before being able to use a new BPL a number of procedures are required such as beam search, detection and signaling to the communications network, configuring of BPL, beam refinement, and/or beam tracking to determine what transmit and receive beams (directions) to use. This may take considerable time.
In scenarios where the radio environment changes fast, and the signal from a used BPL is quickly degraded, there might be too little time to detect and configure a new BPL, in which case there is a risk of losing the connection and thereby also a risk of losing an ongoing communication, e.g. an ongoing call.
According to developments of wireless communications networks an improved beam selection is needed for improving the performance of the wireless communications network.